(1) Field of the Invention
The present invention relates to a traction control apparatus provided in an automotive vehicle for controlling a driving force on driving wheels due to the output torque of an engine of the automotive vehicle so as to prevent the automotive vehicle from undergoing excessive slippage when the vehicle is accelerating on a snowy road or an uphill road.
(2) Description of the Related Art
When an automotive vehicle is running on a snowy road, it is likely that the driving force transmitted from the engine to the driving wheels of the vehicle is greater than the maximum friction force between the wheels and the road. Since the maximum friction force between the wheels and the snowy road is low, the driving wheels of the vehicle undergo excessive slippage, and the vehicle may stall. On the snowy road, the driving force (acting in a direction of vehicle movement) and the side force (acting in a direction lateral to the vehicle movement) are lowered, and the vehicle acceleration is so low that the vehicle running conditions are unstable.
There is a known traction control device for use in an automotive vehicle. In this traction control device, a traction control process is performed in order to increase vehicle acceleration and straight line stability, thus preventing the driving wheels from undergoing excessive slippage when the vehicle starts running or when the vehicle is accelerating.
Herein, a slip ratio can be defined to be a speed difference between a predetermined target rotational speed and the actual rotational speed of the driving wheels of the vehicle. In the known traction control device, a feedback control process for controlling the output torque of the engine is performed so as to maintain the slip ratio at a value smaller than a prescribed speed difference. Hereinafter, this feedback control process performed to control the output torque of the engine is called the traction control process.
Japanese Laid-Open Patent Publication No.60-128055 discloses a traction control device in which a slip ratio is determined in accordance with a speed difference between the driving wheel speed and the vehicle running speed. However, in this traction control device, the output torque of the engine is controlled based on a deviation of the thus determined slip ratio from a desired value.
When the slip ratio is greater than a desired slip ratio, the output torque of the engine is lowered by the traction control device disclosed in the above mentioned publication. Thus, at this time, the driving force transmitted from the engine to the driving wheels is lowered, and the thus lowered driving force acts on the road in the direction of the vehicle movement.
However, in the above traction control device, during the traction control process, the vehicle may stall when the vehicle is accelerating on a snowy road or an uphill road. The driving force characteristics and the side force characteristics when the slip ratio varies are different from each other. The slip ratio is determined in the above traction control device by giving a consideration to the balance of the driving force and the side force. If it is detected that the slip ratio is greater than the desired slip ratio, the driving force is lowered. However, when the vehicle is running on an uphill road, the gravity force may be greater than the driving force. When the vehicle is running on a snowy road, the snow resistance may be greater than the driving force. Thus, in a case in which the driving force is lowered in response to a great slippage, the vehicle may undergo a sudden loss of engine power due to slippage on the snowy or uphill road.